1. Field of the Invention
The present invention relates to zoom lenses, more particularly, though not exclusively, the present invention relates to zoom lenses that can be used as image-taking lenses in imaging apparatuses.
2. Description of the Related Art
In optical imaging systems used for imaging apparatuses such as video cameras using solid-state image-pickup devices, digital still cameras, and silver-salt film cameras using silver-salt films, the desirability of compact-system zoom lenses having optical performance of high resolution have increased.
Since video cameras capable of recording still images of high definition have also been increasingly required, the desirability of lens systems that are compact yet having high-optical performance have increased.
One such zoom lens system that can be used includes four lens groups consisting of a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power, the lens groups being aligned in order from (the anterior position adjacent to) an object side to an image side (the posterior position adjacent to an image taking side). This zoom lens is known as a four-group zoom lens of a so-called rear-focus type in which the second lens group moves so as to change magnification and the fourth lens group adjusts the focus while correcting and/or reducing image-plane fluctuations generated by the magnification changes. Such a zoom lens is discussed in Japanese Patent Laid-Open Nos. 08-304700, 2000-121941, and 2003-295053.
Zoom lenses of the rear-focus type are characterized by small driving power for lens groups and rapid focusing since the lens groups that move to focus are equivalently small and lightweight.
In such zoom lenses of the rear-focus type, a compact zoom lens including a minimum number of lenses has been discussed in Japanese Patent Laid-Open No. 05-027167 and U.S. Pat. No. 6,441,968.
A zoom lens discussed in Japanese Patent Laid-Open No. 05-027167 has a structure that can be used for ⅓-inch and ¼-inch image-pickup devices. In particular, the number of lenses can be reduced by using a plurality of aspherical lenses.
In Japanese Patent Laid-Open No. 05-027167, chromatic aberration may not be sufficiently corrected since the fourth lens group includes only one positive lens. Therefore, a zoom ratio up to 8 times causes no problem, but a zoom ratio higher than or equal to 20 times can lead to large fluctuations of the chromatic aberration during focusing adjacent to the telescopic end.
As in the case of Japanese Patent Laid-Open No. 05-027167, the fourth lens group includes only one positive lens in U.S. Pat. No. 6441968, and thus the chromatic aberration may not be sufficiently corrected. Moreover, the third lens group has a strong refracting power in U.S. Pat. No. 6,441,968, and accordingly, the curvature radii of positive lenses in the third lens group are small. Therefore, when the brightness at a wide-angle end is higher than F1.8, optical performance at the wide-angle end becomes more difficult to be retained satisfactorily. Furthermore, the external diameters of the lenses can become large with the aperture ratio, and the thicknesses of the lenses in the centers can be increased as the curvature radii of the positive lenses become small for ensuring the thicknesses of the lenses in the outer portions, resulting in an increment in size of the whole lens system.
When the refracting powers of the lens groups for changing magnification are enhanced in the zoom lenses, moving distance for a predetermined zoom ratio becomes short. Accordingly, high variable power (high zoom ratio) can be achieved while the total length of the lenses is reduced.
However, when the refracting powers of the lens groups are merely enhanced without appropriately setting the configurations of the lens groups, aberration fluctuation during zooming becomes large, thus making it difficult to achieve excellent optical performance over the whole variable-power range when high variable power is required.